Organic light-emitting devices such as described in U.S. Pat. No. 5,247,190 or in U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference, have great potential for use in various display applications. According to one method, an OLED is fabricated by coating a glass or plastic substrate with a transparent first electrode (anode) such as indium tin oxide (ITO). At least one layer of a thin film of an electroluminescent organic material is then deposited prior to a final layer which is a film of a second electrode (cathode) which is typically a metal or alloy.
The electrode and organic layers used in OLEDs are typically very thin; normally of the order of a few 100 nm and typically around 100 nm and can be flexed without a great risk of damage to the device structure and functioning of the device. By using thin substrates of glass or transparent plastic, formable and/or flexible light sources and displays can be made. For this purpose substrates can be at most a few 100 μm thick.
In order to fabricate OLEDs which have good operating and shelf life it is of utmost importance to protect the active layers of the device, i.e. the electrode and organic layers, from the ingress of ambient species which can react with the active layer and deteriorate device performance, particularly oxygen and moisture. Typically, but not necessarily, an OLED emits light only from one side and this is typically through the transparent substrate and anode. The cathode is typically opaque and is made of a metal or alloy. This opaque side is relatively easy to encapsulate against the ingress of ambient reactive species as, for example, pinhole-free metal foils or metallised plastic foils can be used by, for example, lamination to the cathode.
For OLEDs fabricated on glass substrates the glass itself provides an excellent barrier against the ingress of oxygen and moisture. However, for OLEDs fabricated on transparent plastic foils it is extremely difficult to encapsulate the transparent side against the ingress of ambient reactive species. The oxygen and water permeabilities of even the most impermeable transparent plastic substrates (thin films) presently available are too high to be sufficient as a barrier for long life OLED devices. A simple estimate for this is given for example in K. Pichler, Phil. Trans. R. Soc. Lond. A (1997), Vol. 355, pp 829–842. This situation can be greatly improved by the conductive transparent coating itself, typically an inorganic conductive oxide such as indium tin oxide (ITO). Such ITO coatings on the thin plastic substrates can be very good barriers against the ingress of oxygen and water from outside into the device, as long as the ITO coatings are pinhole-free and defect free. However, these thin ITO (or other conductive oxide coatings) deposited onto thin flexible plastic substrates are prone to “cracking”, if the substrates are not handled with the greatest care. The occurrence of such cracks in the ITO coating creates highly efficient diffusion channels for the ingress of ambient reactive species, just as pinholes in the coating would do. In addition to that, such cracks in the ITO coating may also result in an undesired deterioration of the surface flatness of the coating. This requirement to avoid cracking of the ITO coating puts severe constraints on the handling of the substrates and devices and hence the manufacturing process.
Alternatively, the use as an OLED substrate of thin formable and/or flexible glass with thicknesses of less than 200 μm is possible and even only 30 μm thick flexible glass, which is available commercially, is impermeable to oxygen and water and thus provides excellent barrier properties together with high transparency. Such thin glass is currently available from, for example, DESAG AG, Germany. However, such thin glass, although of a composition and specially manufactured to reduce brittleness, is still extremely difficult to handle and can very easily break if not handled with the greatest care. This puts severe limitations on the use of thin flexible glass as substrates for OLEDs due to the difficulty of manufacturing.